Method for fabricating a magnet structure

ABSTRACT

A magnet structure including a magnet and an integral magnetically permeable concentrator and methods for making the same are provided. The concentrator is comprised of a plastic loaded with one or more magnetically permeable materials to a percentage suitable for providing desired magnetic field characteristics. The magnetic element may be comprised of a conventional magnetic material or a plastic loaded with one or more magnetic materials. An insert molding technique is described in which either the concentrator or magnetic element is prefabricated and inserted into a mold which is subsequently filled with a loaded plastic to provide the other component of the magnet structure. In an alternate fabrication technique, both the concentrator and magnet are sequentially formed by injection molding in the same mold. Also described is a magnet structure including a ring magnet having a central aperture in which a steel rod is disposed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/264,254, filed Mar. 8, 1999 now U.S. Pat. No. 6,278,269.

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

Various applications require the use of magnets designed to create aparticular magnetic field, both in terms of the field pattern andstrength. One such application is a proximity sensor for detecting apassing ferromagnetic article, such as a gear tooth. Conventionalproximity sensors include a magnet and an integrated circuit having aHall device. The integrated circuit is positioned in the magnetic fieldcreated by the magnet. In use, the Hall device generates an electricalsignal related to the strength of the magnetic field normal to the planeof the Hall device. Thus, as a ferromagnetic article moves relative tothe Hall device and the strength of the magnetic field changes, theelectrical signal generated by the Hall device changes.

One way of tailoring the pattern and strength of the magnetic fieldprovided by a magnet structure in order to provide suitable peak andopen circuit characteristics is to use a magnetically permeableconcentrator. The concentrator is positioned in close proximity to themagnet and effectively concentrates the magnetic field as a function ofits material, geometry, and spatial relationship to the magnet.

In one such magnet structure, a magnetically concentrating steel plateis bonded between two semicircular pieces of sintered Samarium Cobalt(SmCo) magnetic material. The resulting structure may then be machinedto provide a desired form factor for use in a particular application.For example, some conventional proximity sensor packages require themagnet structure to have a truncated semicircular cross-section.

The above-described magnet structure suffers certain drawbacks. Inparticular, costly bonding and aligning processes are required toassemble the magnet and concentrator and, even then, tight positiontolerances may be difficult to meet. Further, over time, theconcentrator may move relative to the magnet, thereby adverselyimpacting the device performance. Additionally, there is no easy way to“fine tune” the resulting magnetic field without experimenting withvarious concentrator materials, geometries, and placement relative tothe magnet.

SUMMARY OF THE INVENTION

The invention relates to a magnet structure including a magnetic elementand an integral magnetically permeable concentrator and methods formaking the same. The integral formation of the concentrator and themagnetic element results in a unitary magnet structure in which theconcentrator and magnetic element cannot be separated without destroyingthe structure. The concentrator and magnetic element are held togethereither by an interference fit and/or by a chemical bond. At least one ofthe components is formed by a molding process, with the other componentplaced in (and in one embodiment previously formed in) the mold.

In one embodiment, the concentrator has a substantially planar base witha post extending normal to the base and the magnetic element is providedin the form of a ring magnet having an outer diameter and an innerdiameter defining a central aperture. The concentrator and ring magnetare formed such that the concentrator post extends into the centralaperture of the ring magnet and at least a portion of the concentratorbase is disposed adjacent to a surface of the ring magnet.

The concentrator is comprised of a plastic loaded with one or moremagnetically permeable materials. Suitable plastics includethermoplastics, such as polyamide, polyphenylene sulfide (PPS), andpolyphthalamide (PPA), and thermosets, such as epoxy molding compounds.Suitable magnetically permeable materials include iron, stainless steel,ferrite, and iron oxide. The plastic is loaded with the magneticallypermeable material to a predetermined percentage by weight as thefunction of the desired magnetic field pattern and strengthcharacteristics. As one example, the concentrator is comprised ofpolyamide loaded with iron particles to approximately five percent byweight.

The magnetic element may be comprised of a conventional magneticmaterial, such as sintered Samarium Cobalt (SmCo). Alternatively, themagnetic element may be comprised of a plastic loaded with one or moremagnetic materials. Suitable plastics are as noted above for theconcentrator and suitable magnetic materials include magnetic ferriteand SmCo.

Insert molding techniques are described for providing the integralconcentrator and magnetic element. In accordance with one suchtechnique, one of the components of the magnet structure (i.e., eitherthe magnetic element or the concentrator) is prefabricated and placed ina cavity of a mold and a plastic compound is injected into the mold tocontact at least a portion of the prefabricated component. The plasticcompound is loaded with either magnetically permeable material ormagnetic material, depending on which component is being formed. Forexample, in one embodiment, a sintered SmCo ring magnet is placed into acavity of a mold and a magnetically permeable plastic compound isinjected into the mold so as to contact one surface of the ring magnetand extend into the central aperture of the ring magnet, thereby formingthe concentrator.

In accordance with an alternate insert molding technique suitable forfabricating a device in which the magnet is formed from a magneticallyloaded plastic, a first plastic compound is injected into a mold cavityto form one of the components of the device (i.e., either a magneticplastic compound is injected to form the magnet or a magneticallypermeable plastic compound is injected to form the concentrator).Thereafter, the mold parts are moved relative to one another so as toform a further mold cavity and a second plastic compound is injectedinto this mold cavity to contact at least a portion of the previouslyformed component and form the other component of the structure.

Also described is a magnet structure including a ring magnet and a steelrod concentrator positioned in the central aperture of the ring magnet.In one embodiment, the steel rod is pre-cut to a length substantiallyequal to the height of the magnet and is held in place in the apertureof the ring magnet by a mechanical bonding process, such as with the useof an adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following description ofthe drawings in which:

FIG. 1 is an isometric view of a magnet structure including a ringmagnet and an integral concentrator according to the invention;

FIG. 1A is a cross-sectional view of the magnet structure of FIG. 1taken along line 1A—1A of FIG. 1;

FIG. 1B is a top view of the magnet structure of FIG. 1;

FIG. 2 is a flow diagram illustrating a method for fabricating magnetstructures of the present invention;

FIG. 3 is a flow diagram illustrating an alternate method of fabricatingmagnet structures of the invention;

FIG. 4 is an isometric view of an alternative magnet structure accordingto the invention;

FIG. 4A is a cross-sectional view of the magnet structure of FIG. 4taken along line 4A—4A of FIG. 4;

FIG. 4B is a top view of the magnet structure of FIG. 4;

FIG. 5 is an isometric view of a magnet structure according to a furtherembodiment of the invention;

FIG. 5A is a cross-sectional view of the magnet structure of FIG. 5taken along line 5—5A of FIG. 5;

FIG. 5B is a top view of the magnet structure of FIG. 5;

FIG. 6 is an isometric view of another magnet structure according to theinvention;

FIG. 6A is a cross-sectional view of the magnet structure of FIG. 6taken along line 6A—6A of FIG. 6;

FIG. 6B is a top view of the magnet structure of FIG. 6;

FIG. 7 is a cross-sectional view of a further magnet structure accordingto the invention;

FIG. 8 is an isometric view of still another magnet structure accordingto the invention;

FIG. 8A is a top view of the magnet structure of FIG. 8;

FIG. 9 is an exploded view of a proximity sensor including a magnetstructure according to the invention;

FIG. 10 is a side view of a magnet structure having a front facepositioned adjacent to a Hall element according to the invention;

FIG. 11 illustrates the flux density versus ferromagnetic articleposition relative to a magnet structure of the present invention forvarious air gaps; and

FIG. 12 is an isometric view of another magnet structure according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a magnet structure 10 includes a magnetic element14 and an integral magnetically permeable concentrator 18. Theconcentrator 18 is integral with the magnetic element 14 in the sensethat these components are held together either by an interference fitand/or by a chemical bond. This arrangement is achieved by integralformation of the concentrator and magnet and is in contrast to someconventional magnet/concentrator structures in which the magnet andconcentrator are separately fabricated components which are assembledtogether by a mechanical bonding process and machined to yield a desiredform factor. The illustrative magnetic element 14 is ring-shaped and maybe referred to as a ring magnet. However, it will be appreciated bythose of ordinary skill in the art that magnet structures according tothe present invention may utilize magnetic elements and concentrators ofother geometries and still gain the benefits of the present invention.

The magnet structure 10 is suitable for various applications, such as aproximity sensor, as described in conjunction with FIG. 9 below. Ingeneral, the magnet structure is well suited for use in any applicationin which it is desired to provide a relatively inexpensive magnetstructure which can be easily adapted to provide a desired magneticfield pattern and strength.

The concentrator 18 is comprised of a magnetically permeable materialwhich, due to the fabrication techniques described herein, can bereadily tailored to provide the magnet structure with a predetermined,desired magnetic field pattern and strength. In particular, theconcentrator 18 is comprised of a compound of a plastic material loadedwith particles of one or more magnetically permeable materials. Suitableplastics include: thermoplastics, such as polyamide, polyphenylenesulfide (PPS), and polyphthalamide (PPA), and thermosets, such as epoxymolding compounds. Suitable magnetically permeable materials include:iron, stainless steel, ferrite, and iron oxide.

The relative percentages of the plastic and the magnetically permeablematerial can be varied to suit a particular application. As one example,the concentrator 18 is comprised of polyamide loaded with iron particlesto approximately five percent by weight. As another example, theconcentrator 18 is comprised of polyamide loaded with ferrite particlesto approximately ten percent by weight.

The magnetic element 14 is comprised of one or more magnetic materials,such as sintered Samarium Cobalt (SmCo), or a combination of a plasticand one or more magnetic materials selected to provide predetermined,desired magnetic field characteristics. Suitable plastics includethermoplastics, such as polyamide, polyphenylene sulfide (PPS), andpolyphthalamide (PPA), and thermosets, such as epoxy molding compounds.Suitable magnetic materials include magnetic ferrite and SmCo.Generally, such a magnet is formed by loading the plastic with magnetparticles to a percentage on the order of approximately 60%-70% byweight, such as 63% by weight.

Various techniques are suitable for integrally forming the concentrator18 and magnetic element 14. In the illustrative embodiment, the magnetstructure 10 is formed by an insert molding process, as describedfurther below in conjunction with FIGS. 2 and 3.

Referring also to FIGS. 1A and 1B, cross-sectional and top viewsrespectively of the magnet structure 10 including the concentrator 18and magnet 14 are shown. The ring magnet 14 has a first surface 20, asecond, opposing surface 22, an outer diameter “D”, and an innerdiameter “d” defining a central aperture 30. The concentrator 18 has asubstantially planar base 34 with a diameter “W” and a substantiallycylindrical post 38 extending substantially normal to the base. Aportion of the first surface 20 of the ring magnet 14 is adjacent to aportion of the concentrator base 34, as shown in FIG. 1A.

The ring magnet 14 and integral concentrator 18 are held together by aninterference fit and/or a chemical bond. An interference fit between thetwo components can be achieved in various ways. As one example, theinner diameter “d” of the ring magnet 14 is tapered, as shown in FIG.1A, such that the ring magnet is prevented from slipping off of theconcentrator post. Another technique for achieving an interference fitis to provide a lip on the concentrator which extends into the magnet tointerfere with the magnet, as shown in the embodiment of FIGS. 5-5B. Inapplications in which the magnet 14 is comprised of a magneticallyloaded plastic, the two components (i.e., the magnet and concentrator)may be melted together at an interface to achieve a chemical bond.

In the embodiment of FIGS. 1-1B, the outer diameter “D” of the ringmagnet 14 is substantially equal to the outer diameter “W” of theconcentrator 18. Note however, that other relationships between theouter diameters “W” and “D” are possible, such as is shown in theembodiment of FIGS. 5-5B, and may even be desirable in order tofacilitate manufacture of the device. Further, the concentrator post 38extends into and terminates within the central aperture 30 of the ringmagnet. Here again however, other relationships between the height ofthe concentrator post and the height of the ring magnet are possible andmay be preferable to facilitate certain manufacturing techniques.

Features of the magnet structure 10 can be readily varied withoutdeparting from the spirit and scope of the invention, by suitableselection of the mold(s) used to fabricate the structure and theselected materials. For example, the geometries of the components andthe placement of the components relative to each other can be varied byappropriate design of the mold. Alternative magnet structures accordingto the invention are illustrated in FIGS. 4-8A and in FIG. 12. Themagnet structure 10 may be varied in order to provide different magneticfield characteristics and/or to comply with packaging, space, or othermechanical constraints associated with a particular application.

Referring also to FIG. 2, a flow diagram of an insert molding techniquefor fabricating the magnet structure 10 commences at step 50. In step52, one of the components of the magnet structure (i.e., either themagnetic element 14 or the concentrator 18) which has been prefabricatedis inserted into a cavity of a mold. As one example, the ring magnet isprefabricated by a conventional sintering technique and is comprised ofSmCo.

In step 54, a plastic (i.e., either a thermoplastic or thermoset) isloaded with either magnetically permeable or magnetic particles toprovide the plastic compound. More particularly, in applications inwhich the prefabricated component is the magnet, then the plastic willform the concentrator and is loaded with magnetically permeableparticles; whereas, when the prefabricated component is theconcentrator, the plastic will form the magnet and thus, is loaded withmagnetic particles. In step 56, the plastic compound is injected intothe mold cavity and flows into at least partial contact with theprefabricated component, such as the ring magnet. As will be apparent tothose of ordinary skill in the art, the extent of contact between theplastic compound and the prefabricated component is a function of themold shape and the desired magnet structure.

As noted above, the plastic compound may include a thermoplastic or athermoset plastic. In step 58, the thermoset plastic is cured. This stepis optional since, if the plastic compound injected in step 56 includesa thermoplastic, then curing is not necessary. Finally, the resultingmagnet structure 10 is removed from the mold in step 60, following whichthe process terminates in step 62.

With this arrangement, the magnet structure 10 is fabricated by arelatively inexpensive insert molding technique which permits thechemical composition of the molded component to be readily altered inorder to facilitate fine tuning of the magnetic field characteristics ofthe structure, simply by altering the loading of the plastic. Further,use of an insert molding technique eliminates the alignment andtolerance issues which arise in conventional ring magnet/concentratorstructures, in which the ring magnet and the concentrator are separatelyformed parts which are mechanically bonded together and machined toprovide a desired form factor.

Referring also to FIG. 3, an alternate insert molding technique, whichmay be referred to as a “double shot” technique, is provided forfabricating magnet structures in which both the concentrator and themagnetic element are comprised of a plastic compound. In general, thetechnique includes injection of a first plastic compound to form one ofthe components and subsequent injection of a second plastic compound toform the other component.

The process begins in step 64, after which a plastic compound is formedin step 68. The particular plastic and particles comprising the plasticcompound depend on which component the plastic compound will form. Forexample, if the concentrator is formed first, then the plastic compoundincludes magnetically permeable particles.

In step 70, the plastic compound is injected into a mold cavity formedby mold parts placed in a first position relative to one another and isallowed to harden. Considering the above example, the magneticallypermeable plastic compound is injected and hardens in step 70. In step72, the mold parts are moved to a second position relative to oneanother and to the first formed component so as to form a further moldcavity defining the second component to be formed. Note that in the casewhere the plastic compound includes a thermoset, the plastic is allowedto cure prior to movement of the mold parts.

A second plastic compound is formed in step 74. This plastic compoundincludes either magnetically permeable or magnetic particles, dependingon which component it will form. Still in keeping with the above examplein which the concentrator is formed in step 70, the plastic compoundformed in step 74 includes magnetic particles. In step 76, the secondplastic compound is injected into the further mold cavity. In optionalstep 77, the structure may be heated to cause slight melting at theinterface between the concentrator and magnet element in order toprovide a chemical bond between the components. The magnet structure isthen removed from the mold in step 78, thereby completing thefabrication of the magnet structure in step 82. Here again, if theplastic compound includes a thermoset, then the structure is allowed tocure prior to its removal from the mold.

It will be appreciated by those of ordinary skill in the art that theplastic compounds may be formed prior to the insert molding process. Itwill also be appreciated that the particular sequence of stepsillustrated in the flow diagrams of FIGS. 2 and 3 can be altered in manyinstances.

With this arrangement, a single molding system is used to fabricate boththe concentrator and magnetic element. This technique advantageouslytends to be relatively inexpensive and permits the magnet structure tobe provided with relatively intricate geometries.

Referring to FIGS. 4, 4A, and 4B, an alternate magnet structure 80according to the invention includes a magnetic element 84 in the form ofa ring magnet 84 and a magnetically permeable concentrator 90. Thematerials of the concentrator and ring magnet are as described above inconjunction with FIGS. 1-1B. Further, the magnet structure 80 may beprovided by the insert molding techniques described above in conjunctionwith FIGS. 2 and 3. The magnet structure 80 differs from the magnetstructure 10 in its geometry and the relative placement of theconcentrator 90 and ring magnet 84.

The ring magnet 84 has a first surface 92, a second, opposing surface94, an outer diameter “D”, and an inner diameter “d” defining a centralaperture 96. The concentrator 90 has a substantially planar base 100having a diameter “W”, a substantially cylindrical post 102 extendingnormal to the base, and an annular protrusion 106 also extending normalto the base. The concentrator post 102 and protrusion 106 define anannular depression bordered by the post, the protrusion and a portion ofthe base 100. The ring magnet 84 and concentrator 90 are integrallyformed by one of the above-described techniques such that the ringmagnet 84 occupies this annular depression, as shown.

Referring to FIGS. 5, 5A, and 5B, a further alternate magnet structure120 according to the invention includes a ring magnet 124 and anintegral magnetically permeable concentrator 128. The materials of theconcentrator and ring magnet are as described above in conjunction withFIGS. 1-1B and the magnet structure 120 may be provided by the insertmolding techniques described above in conjunction with FIGS. 2 and 3.

The ring magnet 124 has a first surface 130, a second, opposing surface132, an outer diameter “D”, and an inner diameter “d” defining a centralaperture 136. The concentrator has a lip 148 extending into the magnet,as shown in FIG. 5A. The concentrator 128 has a substantially planarbase 140 with a diameter “W” and a substantially cylindrical post 142extending normal to the base. The ring magnet 124 and concentrator 128are integrally formed such that the concentrator lip 148 results in astep feature in the magnet which prevents the magnet and concentratorfrom becoming separated or moving relative to one another. The outerdiameter “D” of the ring magnet is smaller than the outer diameter “W”of the concentrator, thereby providing a flange 144 at the outer edge ofthe concentrator, as may be desirable to facilitate manufacture of thedevice.

Referring to FIGS. 6, 6A and 6B, a further alternate magnet structure150 according to the invention includes a ring magnet 154 and anintegral magnetically permeable concentrator 158. Here again, thematerials of the concentrator 158 and ring magnet 154 are as describedabove in conjunction with FIGS. 1-1B and the magnet structure 150 may befabricated by the insert molding techniques described above inconjunction with FIGS. 2 and 3.

The ring magnet 154 has a first surface 160, a second, opposing surface162, an outer diameter “D”, and an inner diameter “d” defining a centralaperture 166. The concentrator 158 has a substantially planar base 170with a diameter “W” and a substantially cylindrical post 174 extendingnormal to the base, as shown. The concentrator base 170 further has aplurality of tabs 176 a-176 n, as shown, which may or may not extend tothe post 174, as may be desirable to facilitate manufacture of thedevice.

The ring magnet 154 and concentrator 158 are integrally formed such thatthe concentrator post 174 extends into the central aperture 166 of thering magnet and terminates so as to be flush with the surface 162 of thering magnet, as shown (i.e., the height of the concentrator post 174 issubstantially equal to the height of the ring magnet). Further, theouter diameter “D” of the ring magnet 154 is smaller than the outerdiameter “W” of the concentrator 158 so that the tabs 176 a-176 n extendbeyond the ring magnet, as shown, and as may be desirable to facilitatemanufacture of the device.

Referring to FIG. 7, an alternate magnet structure 190 includes aring-shaped magnet 194 having a pair of protrusions 198 a, 198 b and amagnetically permeable concentrator 192 having complementary pair ofnotches, or indentations 196 a, 196 b. Here again, the materials of theconcentrator and ring magnet are as described above in conjunction withFIGS. 1-1B and the magnet structure 190 may be provided by the insertmolding techniques described above in conjunction with FIGS. 2 and 3.The concentrator and magnet are held together by interference of themagnet protrusions 198 a, 198 b and the concentrator notches 196 a, 196b. It will be appreciated by those of ordinary skill in the art thatalthough the magnet protrusions 198 a, 198 b and the concentratornotches 196 a, 196 b are illustrated to have a squared, step-like shape,alternative complementary shapes, such as a rounded, dome-like shape ora pointed, cone-like shape, are suitable.

Referring to FIGS. 8 and 8A, a magnet structure 200 according to theinvention includes a ring magnet 202 and an integral magneticallypermeable concentrator 204. The magnet structure 200 is comprised of thesame materials and fabricated in the same manner as described above inconjunction with FIGS. 1-3, but differs in that the magnet structure 200has an alternative cross-sectional footprint. In particular, the magnetstructure 200 has a truncated circular cross-section as is desirable forcertain applications.

Referring also to FIG. 9, a proximity sensor assembly 210 includes themagnet structure 200 of the type shown in FIG. 8 and an integratedcircuit 214 containing a Hall device. The assembly 210 further includesa housing 216 having a window 220 and an end cap 224.

In assembly, a portion of the integrated circuit 214 is exposed throughthe housing window 220 for detection of magnetic field changes caused bymovement of a ferromagnetic article past the sensor. More particularly,the magnet structure 200 generates a magnetic field normal to the planeof the Hall device in the chip 214, which magnetic field changes uponthe approach of a ferromagnetic article. The truncated surface 222 ofthe magnet structure 200 is disposed adjacent to conductive leads 218 ofthe integrated circuit 214 and such leads extend through respectivegrooves 226 of the end cap 224. Additional details of the assembly 210are described and shown in U.S. Pat. No. 5,581,179, entitled Hall-effectferrous-article-proximity sensor assembly, which is incorporated hereinby reference in its entirety.

Referring to FIG. 10, the pole configuration of the magnet structures ofthe present invention, such as magnet structure 10 of FIG. 1, is shown.The surface 20 (referred to as front surface 20) defined by an end ofthe concentrator 18 and the magnet 14 presents poles of oppositepolarity. In particular, in the illustrative embodiment, theconcentrator 18 presents a north pole N and the ring magnet 14 presentsa south pole S at the front face 20 of the magnet structure 10. It willbe appreciated by those of ordinary skill in the art that the magneticelement 14 may or may not be a permanent magnet. Likewise, theconcentrator 18 may or may not be a permanent magnet.

Also shown in FIG. 10 is an integrated circuit 250 (like integratedcircuit 214 of FIG. 9) which contains a Hall element 254. As isapparent, the integrated circuit 250 is positioned adjacent to the frontface 20 of the magnet structure 10, with an axis 256 of the magnetstructure normal to the front face 20.

Referring also to FIG. 11, a flux density map (i.e., flux density inGauss versus proximity of the magnet structure to the Hall effect sensorchip 12) associated with the magnet structures of the present invention,such as the magnet structure 10 of FIG. 1, is shown for various airgaps. The pole configuration provided by the magnet structure 10 lowersthe base field (or baseline) of the flux density map by bringing bothpoles of the magnetic field to front surface 20. More particularly, theposition of the opposite poles on the front face 20 serves to short outthe lines of flux when the valley between passing magnetic elements,such as gear teeth, is proximate the integrated circuit 250. Thiscreates a low baseline because the magnetic flux lines are parallel toand below the Hall element 254 in the chip 250. A baseline ofapproximately zero Gauss as measured by the Hall element 254 can beachieved by proper design. When the tooth is present, the Hall elementmeasures a high value. As is apparent, the baseline remains constant andclose to zero even as the air gap between the passing ferromagneticarticle and the integrated circuit 250 varies. This advantageous resultof low baseline substantially independent of air gap is achieved bypresenting opposite poles at the front face 20 of the magnet element 10adjacent to the Hall element 254, as is described in U.S. Pat. No.5,781,005, entitled “Hall-Effect Ferromagnetic-Article-ProximitySensor,” which patent is assigned to the assignee of the presentinvention and incorporated herein by reference in its entirety. Themagnet structures of the present invention achieve this advantageousresult in a cost effective manner by eliminating costly bonding andaligning processes previously required to assemble a magnet andconcentrator.

Referring to FIG. 12, an alternative magnet structure 270 according tothe invention includes a ring magnet 272 and a cylindrical concentrator274 disposed in the central aperture of the magnet. The concentrator 274is provided in the form of a steel rod and the ring magnet 272 may beprovided by various techniques from various materials. As one example,the magnet is comprised of sintered SmCo.

Various techniques are suitable for fabricating the magnet structure270. In the illustrative embodiment, the steel rod 274 is pre-cut to alength substantially equal to the height of the ring magnet 272. Inassembly, the rod is positioned in the central aperture of the magnet sothat the ends of the steel rod are flush with the substantially flat endsurfaces of the ring magnet. A mechanical bond is used to hold theconcentrator 274 and the ring magnet 272 together, such as an adhesive,epoxy, or glue.

Having described the preferred embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may be used. It is felttherefore that these embodiments should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. A method of fabricating a magnet structure,comprising the steps of: inserting an element into a cavity of a mold;and injecting a selected one of a magnetically permeable plasticcompound and a magnetic plastic compound into the mold so as to contactat least a portion of said element, wherein said inserting step and saidinjecting step result in formation of a concentrator in contact with amagnet, so that an end of each of said concentrator and said magnetdefine a front face of said magnet structure, said concentratorpresenting a magnetic field of one polarity at said front face and saidmagnet presenting a magnetic field of the opposite polarity at saidfront face.
 2. The method of claim 1 wherein said element inserting stepcomprises inserting a magnet element into said cavity and said injectingstep comprises injecting a magnetically permeable plastic compound intothe mold.
 3. The method of claim 2 further comprising the step ofcombining a magnetically permeable material and a plastic to form saidmagnetically permeable plastic compound.
 4. The method of claim 3,wherein said combining step further comprises the step of selecting apercentage weight of said magnetically permeable material in response toa desired magnetic field pattern and desired strength characteristics.5. The method of claim 4, wherein said plastic is selected from thegroup consisting of: polyamide, polyphenylene sulfide (PPS),polyphthalamide (PPA), a themoset molding compound, and an epoxy moldingcompound.
 6. The method of claim 4, wherein said magnetically permeablematerial is selected from the group consisting of: iron, stainlesssteel, ferrite, and iron oxide.
 7. The method of claim 1 wherein saidelement inserting step comprises inserting a concentrator element intosaid cavity and said injecting step comprises injecting a magneticplastic compound into the mold.
 8. The method of claim 7, furthercomprising the step of combining a magnetic material and a plastic toform said magnetic plastic compound.
 9. The method of claim 8, whereinsaid step further comprises the step of selecting a percentage weight ofsaid magnetic material in response to a desired magnetic field patternand strength characteristics.
 10. The method of claim 8, wherein saidplastic is selected from the group consisting of: polyamide,polyphenylene sulfide (PPS), polyphthalamide (PPA), and an epoxy moldingcompound.
 11. The method of claim 8, wherein said magnetic material isselected from the group consisting of magnetic ferrite and SmCo.
 12. Themethod of claim 1, further comprising the step of heating the magnetstructure to provide a chemical bond between said element and saidselected one of the magnetically permeable plastic compound and themagnetic plastic compound.
 13. A method of fabricating a magnetstructure, comprising the steps of: injecting a selected one of amagnetically permeable plastic compound and a magnetic plastic compoundinto a first mold cavity formed by mold parts placed in a first positionrelative to one another; moving said mold parts to a second positionrelative to one another to form a second mold cavity; and injecting thepreviously non-selected one of a magnetically permeable plastic compoundand a magnetic plastic compound into said second mold cavity, whereinsaid inserting step and said injecting step result in formation of aconcentrator in contact with a magnet, so that an end of each of saidconcentrator and said magnet define a front face of said magnetstructure, said concentrator presenting a magnetic field of one polarityat said front face and said magnet presenting a magnetic field of theopposite polarity at said front face.
 14. The method of claim 13,wherein said injecting step further comprises the step of combining aselected percentage weight of magnetically permeable material and aplastic to form said magnetically permeable plastic compound in responseto a desired magnetic field pattern and desired strengthcharacteristics.
 15. The method of claim 14, wherein said plastic isselected from the group consisting of polyamide, polyphenylene sulfide(PPS), polyphthalamide (PPA), a themoset molding compound, and an epoxymolding compound.
 16. The method of claim 14, wherein said injectingstep further comprises the step of combining a selected percentageweight of magnetic material and a plastic to form said magnetic plasticcompound in response to a desired magnetic field pattern and desiredstrength characteristics.
 17. A method of fabricating a magnetstructure, comprising the steps of: inserting a rod into a centralregion of a substantially cylindrical mold; and injecting a magneticplastic compound into said mold so as to form a ring-shaped magnetsurrounding and in contact with said rod.
 18. The method of claim 17,wherein the rod is comprised of steel.
 19. The method of claim 17, saidring-shaped magnet has a height substantially equal to the length ofsaid rod.
 20. The method of claim 1, wherein the magnetic field of onepolarity and the magnetic field of the opposite polarity cancel toprovide an essentially zero magnetic field at said front face.
 21. Themethod of claim 13, wherein the magnetic field of one polarity and themagnetic field of the opposite polarity cancel to provide an essentiallyzero magnetic field at said front face.